Lithographic plate and method

ABSTRACT

A LITHOGRAPHIC PLATE COMPRISES A HYDROPHILIC BACKING HAVING AN ALEOPHILIC SOLVENT-REMOVABLE COATING WHICH BECOMES SOLVENT-RESISTANT WHEN HEATED IN REACTIVE ASSOCIATION WITH AN OXIDIZING AGENT. THE COATING COMPRISES A POLYPHENOLIC REDUCING AGENT TOGETHER WITH A LESSER AMOUNT OF CHEMICALLY INERT FILM-FORMING BINDER.

United States Patent Ofice 3,679,414 Patented July 25, 1972 3,679,414 LITHOGRAPHIC PLATE AND METHOD Joan M. W. Mukherjee, St. Paul, Minn., assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn. No Drawing. Filed Aug. 4, 1969, Ser. No. 847,393 Int. Cl. G03c 1/76, 3/00 U.S. Cl. 96-67 8 Claims ABSTRACT OF THE DISCLOSURE A lithographic plate comprises a hydrophilic backing having an oleophilic solvent-removable coating which becomes solvent-resistant when heated in reactive association with an oxidizing agent. The coating comprises a polyphenolic reducing agent together with a lesser amount of chemically inert film-forming binder.

This invention relates to the practice of lithography and in particular to the preparation and imaging of lithographic or planographic offset printing plates. More particularly, the invention relates to lithographic plate constructions comprising a hydrophilic backing, coated with an oleophilic solvent-removable composition containing a polyphenolic reducing agent and a lesser amount of chemically inert film-forming binder. Oxidation of the reducing agent at image-defining areas results in a significant increase in solvent resistance at those areas, permitting the remaining unreacted portions of the coating to be selectively removed to expose the underlying hydrophilic surface.

Oxidation at image areas is accomplished by reaction with metal soap oxidizing agents, of which silver behenate is a presently preferred example. The reactant may be applied only at image areas, e.g. in the form of an ink, but preferably is supplied coextensively with the reducing agent in the same or a separate layer, with subsequent 10- calized image-forming reaction being accomplished by localized heating, catalysis, or other treatment. The silver salt in particular provides a visibly distinct black image so that the subsequent solvent removal of the non-image portions of the coating is facilitated.

Lithographic plates of the invention which contain both silver salt and reducing agent may be imaged in a number of ways. One procedure involves the application of a thermographic heat pattern to the plate coating. In another procedure a light-sensitive intermediate is first eX- posed to a light-pattern and then heated against the plate. In a further and presently preferred method the plate is itself light-sensitive and is exposed to a light-pattern and then heated to develop a solvent-resistant image. Still other procedures may be used. In all cases the still soluble unreacted portion of the coating is washed away to expose the hydrophilic substrate surface, While the reacted portion is solvent-resistant and remains affixed to the substrate to provide oleophilic ink-receptive printing areas.

The heat-reactive oleophilic coatings containing the silver salt and the reducing agent are best applied as liquid compositions in a volatile liquid vehicle which is a solvent for both the reducing agent and the binder. The latter component is present only in minimum amount required to prevent flaking or powdering of the coating, and in minor proportion with respect to the former. Conversion of the reducing agent to a solvent-resistant form then protects the soluble binder from the effects of the solvent.

Polyvinyl butyral is a preferred binder for these compositions and particularly for thel ight-sensitive varieties, since it serves to improve the sensitometric properties of the coatings as well as to provide effective anchorage to the hydrophilic substrate. It is readily soluble in alcohols and alcohol mixtures in which the polyphenol reducing agents are likewise soluble, so that alcoholic solvent mixtures are preferred with these compositions. Other binders which have been found useful include polyvinyl alcohol, chlorinated polyethylene, and copolymer of hydroxypropylmethacrylate and normal butylmethacrylate. In all cases the amount of soluble non-reactive binder is kept as low as possible and in no case amounts to more than about three-fourths the amount by weight of the reducing agent.

Since the film-forming material is initially applied from solution in an organic solvent or solvent mixture, the lithographic master is most conveniently prepared by removal of the background portions of the coating with the same solvent. The solvent power of the original solvent may in some cases be more than adequate for the materials dissolved, and it is therefore frequently found desirable to use less active solvents or solvent mixtures in the development process. Thus, water-miscible alcohol or ketone solvents may be diluted with water to lower the solvent power of the mixture to a point just sufficient for removal of the background portions of the coating, thereby increasing the ability of the reacted image portions to resist the solvent mixture.

The reducing agent is found to undergo substantial increase in molecular weight and to become strongly solvent-resistant when oxidized. Compounds which contain at least two monohydroxyphenyl groups and with hydrogen or a methyl radical in a coupling position, i.e. ortho or para to the hydroxyl radical, and which on oxidation do not form a stable quinoid structure, are suitable. Typical of such compounds are the following:

2,2'-methylenebis (G-tertiarybutylphenol) OH OH 2,2'-methy1enebis (4-methyl-6-tertiarybutylphenol) 1,1 bis-(2-hydroxy-3,S-dimethylphenyl)-3,S,5-trimethylhexane CH3 CH3 2,6 methylenebis (2 hydroxy-3-tertiarybutyl-5-methylphenyl)-4-rnethylphenol CH CH3 CHI 1 4,4'-thiobis (6-tertiarybutyl-o-cresol) OCH:

polyvinyl (2-hydroxy-3-methoxybenzal) repeating structure Ha Hun Ha p-cresol-formaldehyde novolak resin wherein n averages about 4 CH; CH; in Air:

p-cresol-acetaldehyde novolak resin wherein n averages about 4 OH I OHI OH (OHQMOCHOCHQC (CH3):

Ha Ha Ha wherein n is 1 or 2 These poly(monohydroxyphenyl) compounds are characterized by a methyl or hydrogen radical ortho or para to the hydroxyl radical, by solubility in volatile organic solvents, and by the ability to undergo oxidative coupling.

Hydroquinone is an example of a well-known reducing agent for silver which is not useful in the present invention. It contains two hydroxyl radicals on a single benzene nucleus, and on oxidation forms a stable quinoid structure.

Compositions and coatings containing poly(monohydroxyphenyl) compounds as above identified have previously been described; but in no case of which I am aware was the coated sheet material useful for the purposes here set forth. Thus Reitter US. Pat. No. 3,218,166 suggests various phenols, including one example of a polyphenol, namely 4,4 methylenebis (2,6-ditertiarybuty1- phenol), in combination with silver behenate in a coating on paper. The phenolic reducing agent is present in less than the amount needed to produce a dense image. The film-forming binder is present in an amount at least equal to the amount of the reducing agent and preferably in much greater amount. The sheet produces excellent copies 4 of graphic originals when employed as directed in the patent, but does not provide the required differential solubility nor produce offset masters when processed according to the principles of the present invention. Again, Owen US. Pat. No. 2,910,377 describes a heat-sensitive copy-sheet which may contain silver behenate and a reducing agent for silver. One of the reducing agents named, namely 4,4'-dihydroxybiphenyl, is a polyphenol; but this compound forms a soluble stable quinoid structure rather than a solvent-resistant high molecular weight product when oxidized. The copy-sheets of this patent likewise fail to provide an offset master when processed as herein taught.

Since solvent resistance is achieved by oxidation of the reducing agent, it is highly desirable that the oxidation reaction be as complete as possible. For this reason it is preferred to use stoichiometric proportions of the silver salt or other oxidizing agent and the poly(monohydroxyphenyl) reducing agent, although some variation from these proportions may be tolerated, particularly for compositions containing those reducing agents which attain highest molecular weight or otherwise achieve greatest solvent resistance. In some cases the ability of the reducing agent to form a solvent-resistant product is suflicient to permit inclusion of small amounts of other reducing agents which are much less elfective in this respect or which are oxidized to fully soluble end products.

The following specific examples will serve further to illustrate but not to limit the invention.

EXAMPLE 1 A slurry of silver behenate in isopropyl alcohol is homogenized by being forced through an orifice. To 35 grams of the resulting suspension, containing 14.5% by weight of the silver soap, is slowly added 5 ml. of a 4% solution of mercuric bromide in isopropyl alcohol, with constant stirring and under darkroom conditions. Onehalf gram of polyvinyl butyral is added and the mixture is ballmilled for three hours. Separately 1.5 gm. of pcresol-formaldehyde novolak resin and .25 gm. of 1(2H)- phthalazinone is dissolved in 20 ml. of isopropyl alcohol and this solution, together with 2 mil. of .05 solution of sensitizing dye in acetone, is mixed with the soap suspension. Finally 8 gm. of a 5% solution of polyvinyl alcohol in water is added together with 20 ml. of additional water, again with thorough mixing and protected from light.

The mixture is coated on a hydrophilic silicate-treated aluminum panel at a coating ofiice of three mils, and dried. The sheet is exposed to a light-image at foot candle seconds and a visible image is developed in the light-struck areas by heating at C. for a few seconds. The plate is then carefully scrubbed with a 1:1 mixture of isopropyl alcohol and water. The unexposed and undarkened portions of the coating are completely removed to expose the hydrophilic substrate. The black ink-receptive image areas remain on the plate, which is then placed on the press, inked, and used in printing a number of copies by the lithographic offset process.

Light-sensitive heat-developable coated sheet materials of similar composition but employing different reducing agents and which do not possess the separability characteristics of the coatings here described are the subject of Morgan et al. US. application Ser. No. 693,714 filed Dec. 27, 1967, now US. Pat. 3,457,075.

The novolak resin is a thermoplastic alcohol-soluble acid-catalyzed resinous condensation product of one mol of p-cresol and 0.8 mol of formaldehyde. The sensitizing dye is 3-allyl-5-[3-ethyl-(Z-naphthoxazoylidene)-ethylidene]-l-phenyl-2-thiohydantoin. The silver behenate, prepared by precipitation with silver nitrate from an aqueous solution of the sodium soap of commercial behenic acid, analyzes 25.2% silver.

Substitution for the novolak resin of 2 gm. of the compound 1,1 bis (2 hydroxy 3,5 dimethylpheny1)- 3,5,5 trimethyl-hexane gives a sensitive coating which produces a visible and solvent-resistant image when exposed to 1000 foot-candles and heated at 145 C. and provides increased press life as compared with the novolak resin coating.

EXAMPLE 2 A mixture of 35 grams of the silver behenate homogenizate and 5 ml. of the mercuric bromide solution is prepared as described in Example 1. To this solution is added a solution containing, in ml. of isopropyl alcohol, 020 gm. of phthalazinone, 0.5 gm. of polyvinyl butyral, 2 ml. of .05% solution of sensitizing dye in acetone, and one gram each of the compounds 1,1-bis-(2-hydroxy-3,5-dimethylphenyl) 3,5,5 trimethylhexane, 2,2 methylenebis (4 methyl 6 tertiary-butylphenol), and 2,2- methylenebis (4,6 ditertiarybutylphenol). The wellmixed composition is immediately coated on silicated aluminum at a wet thickness of three mils. The coating is dried and is then exposed to a light-image for 50 foot candle seconds followed by heating for 10 seconds at 122 C. The non-light-exposed areas are preferentially darkened and rendered solvent-resistant; very little darkening is observed at the light-struck image areas. Washing with a mixture of equal parts isopropyl alcohol and water removes the coating at the light-exposed areas, leaving the darkened and ink-receptive background areas. The resulting plate is placed on the press, inked, and used in making copies by oflset printing.

EXAMPLE 3 A mixture of 33 grams of the silver behenate homogenizate described under Example 1 and 5 ml. of a four percent solution of mercuric bromide in isopropyl alcohol is ball milled until uniform. To the mixture is added a solution of .25 gm. phthalazinone and 1.5 gm. of pcresol-formaldehyde novolak resin in 20 ml. of ethyl acctate, together with 2 ml. of a .05% solution of sensitizing dye in acetone. The resulting mixture is coated on silicated aluminum at a coating thickness of 3 mils and dried, all under darkroom conditions.

Separately a thin heat-resistant polyester film is coated with a solution of .2 part by weight of 4-methoxy-1-naphthol, .088 part of erythrosin and 10 parts of ethyl cellulose in 90 parts of methylethyl ketone, applied at a three mil thickness and dried, to form a light-desensitizable intermediate as described in Workman US. Patent No. 3,094,417.

- The intermediate is placed against a printed original and exposed to light to the extent just suflicient to desensitize the coating at the non-printed areas. It is then removed from the original and placed against the coated surface of the plate, and the composite is heated for 10 seconds at 130 C. Removal of the intermediate discloses a black image on the plate corresponding to the printed image of the original. The plate is carefully washed with a mixture of equal parts of isopropyl alcohol and water. The unprinted background portions of the coating are thereby removed, exposing the hydrophilic substrate and leaving the solvent-resistant ink-receptive image areas. Copies are produced from the plate by the lithographic offset printing process.

EXAMPLE 4 A blend of 25 gm. of a homogenizate of 20 parts silver behenate in 80 parts 2-propanol, 0.5 gm. of polyvinylbutyral, and 0.25 gm. of phthalazinone is prepared by grinding in a ball mill. A solution of 1.5 gm. of p-cresolformaldehyde novolak resin and 0.5 gm. of phenolblocked polyol-isocyanate resin (Mondur S) in 10 ml. of Z-propanol is added. The smooth mixture is coated through a 3 mil coating orifice onto a polyester film which has previously been given a hydrophilic surface coating of colloidal silica in Vinylite VAGH partially hydrolyzed copolymer of vinyl chloride and vinyl acetate. The

dried coating is non-light sensitive but heat-sensitive. A graphic original printed with black infra-red-absorptive characters on a thin white paper is placed against the coated plate with the unprinted surface in contact with the coating and is then briefly exposed to intense radiation rich in infra-red. Removal of the original reveals a black copy of the original image. The plate is washed with a mixture of isopropyl alcohol and water to remove the coating and expose the hydrophilic undersurface at background areas, leaving the solvent-resistant and inkreceptive image. Copies are produced by the lithographic offset process.

Equally effective lithoplate masters are prepared in the same manner on a paper backing having a hydrophilic surface coating of clay and insolubilized casein or carboxymethylcellulose binder.

EXAMPLE 5 An aluminum plate having a hydrophilic silicate-treated surface is coated with a solution of one gram of polyvinyl butyral resin, two grams of 4,4'-thiobis-(6-tertiarybutylorthocresol), and 0.25 gram of phthalazinone in 15 ml. of methanol, applied at a thickness of two mils, and dried.

Various markings are applied to the coated surface using inks containing respectively silver behenate, silver phthalate, mercuric stearate, ceric behenate, and ferric stearate as suspensions in acetone or methanol. Preferably, polyvinyl butyral is incorporated in such inks as a binder and viscosity increasing agent. The plate is heated and the marked areas change to a black color. Washing with a mixture of water and isopropyl alcohol removes the unmarked portions to expose the hydrophilic sub-surface. The marked portions are oleophilic and the plate is useful as a lithographic master.

EXAMPLE 6 A mixture of 25 gm. of 20% silver behenate suspension in isopropyl alcohol, one gm. of polyvinyl butyral resin, two gm. of 4,4'-thiobis(6 tertiarybutyl-o-cresol) and 20 ml of the isopropyl alcohol is applied to a silicated aluminum panel and dried. Markings are applied to the surface of the coating with a marking fluid consisting of a solution of phthalazinone (a catalyst for the oxidation reaction) in a volatile solvent. Heating of the marked plate results in development of a black image and of suflicient solvent resistance to permit separation of image and background areas by washing with isopropyl alcoholwater mixtures. The plate is useful as a lithographic master.

What is claimed is as follows:

1. A lithographic plate having a thin oleophilic ink-receptive layer on a hydrophilic substrate, said oleophilic layer being readily removable by washing with a solvent mixture of organic solvent from the group consisting of water-miscible alcohols and ketones and water, said layer containing as an essential ingredient an oxidizable, low molecular weight poly(monohydroxyphenyl) aromatic organic compound soluble in water-miscible alcohols and ketones and having a hydrogen or methyl radical in ortho or para position to the hydroxyl radical and which on oxidation does not form a stable quinoid structure, together with a lesser amount by weight of a chemically inert filmforming binder, said coating, when briefly heated with a quantity of water-insoluble silver soap just suflicient to oxidize all of said compound and in the presence of phthalazinone as a catalyst for such oxidation reaction, being rendered resistant to attack by said solvent mixture.

2. The lithographic plate of claim 1 wherein said layer contains said quantity of said silver soap.

3. The lithographic plate of claim 2 wherein said layer contains said catalyst for the oxidation-reduction reaction.

4. The lithographic plate of claim 2 wherein about one to about ten percent of said silver soap is converted to photosensitive silver halide.

5. The lithographic plate of claim 2 wherein said organic compound is a methylene bis 0- or p-tertiaryalkyl phenol.

6. The lithographic plate of claim 2 wherein said organic compound is a low molecular weight p-cresol-aldehyde novolak resin.

7. The lithographic plate of claim 1 wherein said substrate is a metal plate having a hydrophilic surface.

8. The lithographic plate of claim 1 wherein said substrate is a thin flexible sheet material having a hydrophilic water-resistant surface coating comprising a hydrophilic mineral powder in a hydrophilic binder.

References Cited UNITED STATES PATENTS 3,168,864 2/1965 Brandl 61 al. 101-467 5 2,663,657 12/1951 Miller 6161. 117-36.8

3,218,166 11/1965 Reitter 96-67 DAVID KLEIN, Primary Examiner Us. 01. X.R. 

